Development and characterisation of injectable calcium phosphate cements for use in vertebroplasty

The global objective of this thesis was to understand how the starting components of brushite cements influence the cement properties relevant for its use in vertebroplasty. Therefore, this work focussed on the following cement properties : mechanical strength, X-ray opacity and heat release upon setting. To carry out the global objective, a test protocol was first developed to characterise the mechanical properties of calcium phosphate cements. The Mohr's circles representation allowed to understand that for a material like our cement, ultimate stress measured with the Brazilian test can only underestimate tensile strength because the compressive/tensile strength ratio is lower than 8. Nevertheless, the Brazilian test remains a test that is easy to use with brittle materials and requires simple sample shape. Therefore, it was considered as an appropriate test for our cements, and was used as a way to measure the indirect tensile strength of the cements tested in the run of this work. Refining two of the starting powders, MCPM and CSH, allowed to increase significantly both compressive and diametral tensile strengths. Milling of these powders provided a five-fold increase in axial compression, and a three-fold increase in diametral compression. The mechanical strengths of cements made with milled MCPM and CSH were the highest measured in this thesis work : 4.1 MPa diametral tensile strength and 22.1 MPa compressive strength. The mechanical properties of the cement were found to be strongly dependent on both CSH granulometry and chemical purity. A computer model was developed to calculate the linear X-ray attenuation coefficient of any material of known composition. It was then validated by experimental measurements. The standard linear X-ray attenuation coefficient a cement should exhibit to be used in vertebroplasty was found to amount 2.47 cm-1 for the conditions simulated. It was calculated from an acrylic cement composition considered a standard in vertebroplasty. Calculation of the attenuation coefficient of a brushite cement revealed that this material is not radiopaque enough for safe use in vertebroplasty. Therefore, we added iodinated molecules to a brushite cement formulation so as to produce the required X-ray opacification. We found that incorporation of Iopentol, a contrast media commonly used in X-ray imaging applications, does not impair the mechanical properties of the cement and does not alter the setting time of the reaction either. Finally, a custom-made calorimeter was developed for measurement of kinetic and thermometric parameters of cements. One single measurement allowed calculation of the maximum reaction rate of the setting reactions, the working and setting times of the cement pastes, the degree of conversion of the cements, and the temperature increase induced by the setting reaction. The degree of hydration of the calcium sulfate was found to have a significant influence on the kinetics of the setting reaction. The reaction is accelerated by calcium sulfate dihydrate because it provides nuclei for heterogeneous precipitation of brushite. In contrast, calcium sulfate hemihydrate was found to slow down the reaction by releasing sulfate ions in the mixing liquid. As the reaction is faster when CSD is used, heat production occurs faster and accumulates in the system. As a consequence, the temperature elevation during setting is more important.